1. Field of the Invention
The present invention relates to composite fibers, and more particularly to a method of producing composite staple organic fibers reinforced with fine and short inorganic fibers.
According to the present invention, the composite staple fibers have superior properties, such as strength, rigidity, electrical conductivity, thermal conductivity, frictional characteristics, and wear resistance of the fine and short inorganic reinforcing fibers, are utilized for unwoven fabric or felt through processing steps, and are added to and mixed in a resin (plastic), cement or the like, to improve the strength, rigidity, and electrical conductivity thereof.
2. Description of the Related Art
Fibers or composite fibers are hereinafter discussed from the viewpoint of, for example, the strength and electrical conductivity of the fibers. Carbon fibers made from raw fibers, such as PAN fibers and pitch fibers are well-known as electric conductive fibers, and have a high electrical conductivity, i.e., a specific resistance (.rho.) of 1500 to 2000 .mu..OMEGA.cm. Since the conductivity increased in accordance with the level of graphitization of the carbon fiber, a specific resistance (.rho.) of 800 to 1000 .mu..OMEGA.cm can be attained. Furthermore, since the carbon fibers have a high strength and a high elastic modulus, they are widely used as a reinforcing material for, e.g., sports equipment such as golf clubs, fishing rods, and tennis rackets. The production process of the carbon fibers is complicated, and thus these fibers are very expensive.
Recently a method of cheaply and efficiently producing carbon fibers by a vapor phase growth process was developed. The carbon fibers produced by vapor phase growth (hereinafter called VGCF's) have superior mechanical properties, such as strength and elastic modulus, and a good electrical conductivity. The level of graphitization of the VGCF's can be increased to obtain a specific resistance (.rho.) of 100 .mu..OMEGA.cm or less and to improve the mechanical properties, but when the VGCF's are produced in a relatively short period, they have a diameter of less than 1 .mu.m and a short length. If the diameter and length of the VGCF's are to be made thicker and longer, a long growth period is necessary, which raises the production cost.
It is well-known that an electrically conductive powder (filler) such as metal powder and carbon black is mixed with an organic material (synthetic resin) to make the organic material conductive and then the organic material is spun into composite fibers but the mechanical properties of the obtained fibers are inferior to those of the organic material without the filler. To obtain a practical electrical conductivity of the fibers, the mixture must contain a large amount of the filler, and as a result, the mechanical properties of the obtained fibers are reduced. If the carbon fibers made from PAN fibers or pitch are mixed into the organic material (resin) and then the organic material is spun into composite (carbon-fiber-reinforced) fibers, i.e., CFRP's, since each of the carbon fibers has a diameter of 7 .mu.m or more, each of the composite fibers has a diameter of several tens of micrometers or more, and thus has a disadvantage of rigidity. A clad-core simultaneous drawing method used for obtaining very fine fibers can be applied to the formation of composite organic fibers reinforced with the carbon fibers made from PAN fibers or pitch. In this case, the carbon-fiber-reinforced fibers (core) are clad with another organic (clad) layer and then the clad filament is drawn to form a further fine fiber, but the obtained fiber is expensive.
The present inventors were aware of experiments carried out to obtain composite organic fibers having an improved strength and electrical conductivity by using fine and short inorganic fibers as a filler mixed with an organic material, and that as a result, the VGCF's have a much higher electrical conductivity than that of carbon black, have good drape to generalpurpose resin, and do not decrease a flowability of a resin containing the VGCF's, during injection molding, and it was found that the resin containing the VGCF's can be easily spun to form fibers.
The present applicant filed Japanese Patent Application No. 62-275508 on Oct. 30, 1987 (corresponding to U.S. Pat. Ser. No. 264109 filed on Oct. 28, 1988), proposing composite organic fibers based on the above finding, having a high strength, a superior electrical conductivity, and inexpensive.
The proposed composite organic fibers have a relatively long fiber length, and thus problems must be taken into consideration, such as breaking, which complicates the spinning process. To obtain staple fibers from the proposed fibers, the fibers must be chopped, and therefore, the production of staple fibers becomes very complicated.